This invention relates generally to memories that use phase-change materials.
Phase-change materials may exhibit at least two different states. The states may be called the amorphous and crystalline states. Transitions between these states may be selectively initiated. The states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. The amorphous state involves a more disordered atomic structure and the crystalline state involves a more ordered atomic structure. Generally, any phase-change material may be utilized; however, in some embodiments, thin-film chalcogenide alloy materials may be particularly suitable.
The phase-change may be induced reversibly. Therefore, the memory may change from the amorphous to the crystalline state and may revert back to the amorphous state thereafter or vice versa. In effect, each memory cell may be thought of as a programmable resistor, which reversibly changes between higher and lower resistance states.
In some situations, the cell may have a large number of states. That is, because each state may be distinguished by its resistance, a number of resistance determined states may be possible allowing the storage of multiple bits of data in a single cell.
A variety of phase-change alloys are known. Generally, chalcogenide alloys contain one or more elements from column VI of the periodic table. One particularly suitable group of alloys are GeSbTe alloys.
A phase-change material may be formed within a passage or pore defined through a dielectric material. A phase-change material may be sandwiched between lower and upper electrodes.
A buried wordline structure may be utilized to provide signals to the lower electrode of a phase-change material memory cell. These signals may include programming signals to change the programming state of the cell or read signals to read the current state of the phase-change material.
A buried wordline may form a diode in a substrate under the lower electrode. The diode necessarily has a certain reverse bias leakage current to the substrate. In particular, the diode is formed of a P+ layer over an N+ layer over a P type substrate. When the N+ type layer is reverse biased between the N type layer and the substrate, an N+/N−/P− diode is formed whose reverse leakage current may be substantial.
Generally, the N+ layer must be relatively narrow resulting in higher wordline resistance. These structures may have relatively high resistance and at times may have substantial leakage currents.
Thus, there is a need for way to reduce the leakage currents of phase-change memory cells.